Energy harvesting refers to a process of capturing energy from external sources, for example, kinetic energy, sunlight, wind, hydraulics, etc. Energy that is harvested from different sources is typically bountiful, and is present regardless of whether energy harvesting takes place. The harvested energy is often converted to electricity to power electronic devices. Most existing energy harvesting technologies focus on small systems that cannot provide sufficient power to commonly used electronic devices. In an era that emphasizes green technology, there is a need for finding new ways to save and reuse energy, while also making it affordable to do so. For devices such as smartphones, energy harvesting technologies with sufficient power would mean that people no longer need to tether their smartphones to sockets to charge their smartphones. Instead, people can use an energy harvesting technology to charge their smartphones on the go. Other devices, for example, communication radios and flashlights, would also benefit from energy harvesting technologies. In places where power sources may be unavailable, for example, underground mines, deserts, and remote areas, energy harvesting technologies could sustain small electronic devices indefinitely.
Energy harvesting technologies are used in many existing systems, for example automatic watches. An automatic watch captures a small amount of the energy generated during movement of a wearer's arm and uses the captured energy to wind a spring within the automatic watch. The spring slowly releases the energy, thereby powering gears and clock hands of the automatic watch. Therefore, the automatic watch does not rely on batteries or manual winding. The automatic watch functions as long as the wearer's arm moves. However, the automatic winding mechanism is not suitable for large devices since the automatic winding mechanism provides insufficient power of, for example, few milliwatts.
Conventional energy harvester systems use micro-electrostatic vibrations to generate electricity. The reduction in size and power consumption of complementary metal-oxide semiconductor (CMOS) circuitry has led to research based on wireless sensor networks. Proposed networks include thousands of small wireless nodes that operate in a multi-hop fashion, replacing long transmission distances with multiple low power and low cost wireless devices. The result is a creation of an intelligent environment that responds to its inhabitants and ambient conditions. Wireless devices being designed and built for use in such environments typically run on batteries. However, as networks increase in number and devices decrease in size, the replacement of depleted batteries is not practical. The cost of replacing batteries in a few devices that make up a small network about once a year is feasible. However, the cost of replacing thousands of devices annually, some of which are in areas difficult to access, is not practical. Another approach would be to use a battery that is large enough to last the entire lifetime of a wireless sensor device. However, a battery large enough to last the lifetime of the wireless sensor device would dominate the overall system size and cost, and thus is not practical. There is a need for alternative methods of powering devices that make up wireless networks. A conventional energy harvester system converts micro electrostatic vibration to electricity using a microelectromechanical systems (MEMS) fabrication technology with an output power density of, for example, about 116 μW/cm3. However, the MEMS based energy harvester system is expensive and generates low power.
Other conventional energy harvesting systems are compatible with mobile devices. The mobile devices domain comprises, for example, sound energy harvesting, electro-magnetic wave energy harvesting, and photo cell energy harvesting, for example, solar cell energy harvesting. When a person speaks over a mobile device, for example, a phone, sound energy is used to vibrate a coil or a magnet in the phone to generate electricity. Electromagnetic waves are ubiquitous and received by a coil with an iron core to generate electricity. Photons, for example, from the sun or a lamp are also ubiquitous. Photodiodes mounted on the surface of a mobile device receive light and generate electrical current. Combining these energy harvesting techniques with mechanical energy harvesting techniques reduces the size of an energy harvester and provides sufficient energy to power small portable devices at the same time. However, the energy generated is insufficient for large devices.
Another conventional energy harvesting system includes a device that generates electricity from mechanical energy when embedded in a vibrating medium. Supplying power to remote microsystems that have no physical connection to the outside world is difficult, and using batteries is not always appropriate. A micro generator generates electricity from mechanical energy when embedded in a vibrating medium. This micro generator has dimensions of, for example, about 5 mm×5 mm×1 mm. Analysis predicts that power produced is proportional to a cube of the frequency of vibration, and that to maximize power generation, the mass deflection should be as large as possible. Power generation of, for example, about 1 μW at 70 Hz and 0.1 mW at 330 Hz are predicted for a typical device, assuming a deflection of 50 μm.
In another conventional energy harvesting system, a generator produces sufficient electricity from random, ambient vibrations to power a wristwatch, a pacemaker, or a wireless sensor. Energy harvesting devices created in this manner provide renewable electrical power from arbitrary, non-periodic vibrations. Non-periodic vibrations are obtained, for example, from traffic driving on bridges, machinery operating in industries, and humans moving their limbs. According to a research study, a generator harnesses energy from nearby vibrations using piezoelectric materials. The piezoelectric materials create a charge when stressed. The piezoelectric materials allow each generator of one cubic centimeter in volume to create power of, for example, about 0.5 milliwatts, which can potentially be used to drive small autonomous devices, for example, pace makers. The conventional energy harvesting systems using piezoelectric materials generate insufficient power to power a standard portable electronic device. Moreover, the piezoelectric materials are expensive. In an experimental study, micro electrostatic vibration-to-electricity converters using the microelectromechanical systems (MEMS) fabrication technology with an output power density of, for example, about 116 μW/cm3 are designed.
Another conventional energy harvester system utilizes sensitive vibrations to generate electrical energy. A vibration energy harvester is capable of using mechanical energy to harvest useful energy. This conventional vibration energy harvester that generates electrical energy from mechanical energy utilizes a complex mechatronic device, which includes a precise mechanical part, an electromagnetic converter, power electronics for power management, and a load. The energy generated by the above systems is insufficient for large devices.
Hence, there is a long felt but unresolved need for an affordable and easily available energy harvester system that generates sufficient power for powering electronic devices, for example, smartphones, communication radios, flashlights, etc.